From: Vincenzo Frascino <vincenzo.frascino@arm.com>

On arm64 the TCR_EL1.TBI0 bit has been always enabled hence
the userspace (EL0) is allowed to set a non-zero value in the
top byte but the resulting pointers are not allowed at the
user-kernel syscall ABI boundary.

With the relaxed ABI proposed in this set, it is now possible to pass
tagged pointers to the syscalls, when these pointers are in memory
ranges obtained by an anonymous (MAP_ANONYMOUS) mmap().

Relax the requirements described in tagged-pointers.rst to be compliant
with the behaviours guaranteed by the ARM64 Tagged Address ABI.

Cc: Will Deacon <redacted>
Cc: Andrey Konovalov <redacted>
Cc: Szabolcs Nagy <redacted>
Cc: Kevin Brodsky <redacted>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
[catalin.marinas@arm.com: minor tweaks]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 Documentation/arm64/tagged-pointers.rst | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
index 2acdec3ebbeb..82a3eff71a70 100644
--- a/Documentation/arm64/tagged-pointers.rst
+++ b/Documentation/arm64/tagged-pointers.rst

@@ -20,7 +20,8 @@ Passing tagged addresses to the kernel
 --------------------------------------
 
 All interpretation of userspace memory addresses by the kernel assumes
-an address tag of 0x00.
+an address tag of 0x00, unless the application enables the AArch64
+Tagged Address ABI explicitly.